The reduction of the group --NO.sub.2 by the pair Fe--Fe.sup.++ in a sulfuric acid or acetic acid medium is known, but the weight of the reagent used is about three times that of the nitro derivative to be reduced. A large amount of solid residue to be eliminated results and it is necessary to rectify the liquid containing the amine to obtain a pure product. The yield is on the order of 80%.
It is also possible to perform a catalytic hydrogenation, for example, on Raney nickel in a methanol medium under 6 MPa at 40.degree.-45.degree. C.
In this case also, the yield does not exceed 80%; the secondary reactions are numerous, involving the formation of light amines and heavy residue which must be separated from the desired amino alcohol by several successive rectifications which involve considerable investment and energy consumption; further, it is not possible to avoid the formation of the N-CH.sub.3 derivative which is then difficult to separate from the desired nitro derivative.
A process of electrochemical reduction has already been described in U.S. Pat. No. 2,485,982 according to which the operation is performed in an aqueous hydrochloric or sulfuric solution in an electrochemical cell provided with a porous porcelain diaphragm; an aqueous solution of amino alcohol hydrochloride and sulfate is obtained which then must be neutralized and/or precipitated to obtain the amine. In addition to the raw material derivative, the acid and neutralization or precipitation reagents are consumed which then must be rejected into the environment.
In French application No. 2,577,242 the applicant described a process for reduction of nitro alcohols into amino alcohols in a diaphragm cell in which the anode and cathode compartments are separated by a cation-exchange membrane; the catholyte consists of an aqueous sulfuric solution of nitro alcohol and the anolyte of a dilute sulfuric acid solution.
It is possible to represent this transformation overall by two reactions: EQU RNO.sub.2 +4e-+4H.sup.+ --R-NHOH+H.sub.2 O (1) EQU R NHOH+2e-+2H.sup.+ --R-NH.sub.2 +H.sub.2 O (2)
Reaction (1) is performed at an electronegative potential close to -0.8 volts. It can be used on a large number of materials with a slight hydrogen overvoltage such as stainless steel, copper, or nickel.
Reaction (2), on the other hand requires a potential close to or greater than -1.5 volts; it can be performed only on materials with a large oxygen overvoltage to favor the reduction of the --NHOH group relative to that of the proton. Choice of the material is then limited to four or five metals such as mercury, lead, zinc, cadmium, tin and materials with a carbon base such as graphite and vitreous carbon.
Good results have been obtained on lead and on mercury amalgams, and on copper, nickel and lead.
But this process has several drawbacks. It was found that it was not possible totally to avoid corrosion of the cathode, which results in contamination of the amino alcohol obtained with traces of mercury or of lead toxic cations. Further, a deactivation of the cathode surface was observed; in the best case this deactivation occurs after several dozen hours of operation and thus makes the process unsuitable for industrial use.